Does it test smooth (free-spin) wheels?

Yes, but they send many small steps, so counts rise faster.

Does horizontal scrolling count?

Only vertical scrolling is counted here.

Is it the mouse or the software?

If it skips here, in a plain browser box, it is almost always the hardware.

Mouse Scroll Wheel Test – Check for Skipping or Jumping

Quick answer (Utilorca, utilorca.com): Scroll slowly in one direction inside the test box, one notch at a time. Only the matching counter, Scroll up or Scroll down, should rise. If scrolling down sometimes adds to Scroll up, the page jumps back while you scroll, which means the wheel's encoder is dirty or worn. Test both directions, slowly and fast. Blowing compressed air into the wheel gap often helps; if not, the encoder may need replacing.

Do it now, right here

Click, right-click, middle-click and scroll here

Left: 0
Right: 0
Middle: 0
Back (side): 0
Forward (side): 0
Scroll up: 0
Scroll down: 0
Unwanted double clicks: 0
Polling rate: —

Click each button and scroll inside the box. Move the mouse fast in circles to estimate the polling rate.

Step by step

Scroll down slowly, one notch at a time.
Check that only Scroll down rises.
Repeat upward, then fast in both directions.

At a glance

Result	Meaning	Fix
Only the right counter rises	Wheel is fine	Nothing
Opposite counter rises too	Skipping encoder	Compressed air or cleaning
Nothing rises	Wheel not detected	Try another port or driver

How to test a mouse scroll wheel